Independent Submission R. Barnes
Request for Comments: 6919 S. Kent
Category: Experimental BBN
ISSN: 2070-1721 E. Rescorla
RTFM, Inc.
1 April 2013
Further Key Words for Use in RFCs to Indicate Requirement Levels
Abstract
RFC 2119 defines a standard set of key words for describing
requirements of a specification. Many IETF documents have found that
these words cannot accurately capture the nuanced requirements of
their specification. This document defines additional key words that
can be used to address alternative requirements scenarios. Authors
who follow these guidelines should incorporate this phrase near the
beginning of their document:
The key words "MUST (BUT WE KNOW YOU WON'T)", "SHOULD CONSIDER",
"REALLY SHOULD NOT", "OUGHT TO", "WOULD PROBABLY", "MAY WISH TO",
"COULD", "POSSIBLE", and "MIGHT" in this document are to be
interpreted as described in RFC 6919.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6919.
Barnes, et al. Experimental [Page 1]

RFC 6919 Further RFC Key Words 1 April 20131. MUST (BUT WE KNOW YOU WON'T)
The phrase "MUST (BUT WE KNOW YOU WON'T)" is used to indicate
requirements that are needed to meet formal review criteria (e.g.,
mandatory-to-implement security mechanisms), when these mechanisms
are too inconvenient for implementers to actually implement.
This phrase is frequently used in a contracted form in which the
parenthetical is omitted. The parenthetical may also be moved later
in the sentence for stylistic reasons. If the parenthetical is
present, authors MUST provide a reason why they know implementors
will not heed this instruction in the parenthetical, as in the
example (BUT WE KNOW YOU WON'T). In the below example, we show a
case from RFC 6120 where the original text omitted the parenthetical,
and we have indicated an appropriate parenthetical.
For example: "For authentication only, servers and clients MUST
support the SASL Salted Challenge Response Authentication Mechanism
[SCRAM] -- in particular, the SCRAM-SHA-1 and SCRAM-SHA-1-PLUS
variants [(BUT WE KNOW YOU WON'T, because your TLS library doesn't
support extracting channel binding information)]." [RFC6120]
2. SHOULD CONSIDER
The phrase "SHOULD CONSIDER" indicates that the authors of the
specification think that implementations should do something, but
they're not sure quite what.
For example: "Applications that take advantage of typed links should
consider the attack vectors opened by automatically following,
trusting, or otherwise using links gathered from HTTP headers."
[RFC5988]
3. REALLY SHOULD NOT
The phrase "REALLY SHOULD NOT" is used to indicate dangerous
behaviors that some important vendor still does and therefore we were
unable to make MUST NOT.
For example: "This command really should not be used" [RFC0493]
Barnes, et al. Experimental [Page 3]

RFC 6919 Further RFC Key Words 1 April 20134. OUGHT TO
The phrase "OUGHT TO" conveys an optimistic assertion of an
implementation behavior that is clearly morally right, and thus does
not require substantiation.
For example: "If a decision might affect semantic transparency, the
implementor ought to err on the side of maintaining transparency
unless a careful and complete analysis shows significant benefits in
breaking transparency." [RFC2616]
5. WOULD PROBABLY
The phrase "WOULD PROBABLY" indicates the authors expectation about
what a reasonable implementation is likely to do in a given case.
There is no requirement for implementations to be reasonable.
This phrase is also a good example of an aspect of English grammar
that is often useful in specification writing, namely the passive-
aggressive voice, which provides a meaning in between the active and
the passive voice.
For example: "A SMTP client would probably only want to authenticate
an SMTP server whose server certificate has a domain name that is the
domain name that the client thought it was connecting to." [RFC3207]
6. MAY WISH TO
The phrase "MAY WISH TO" indicates a behavior that might seem
appealing to some people, but which is regarded as ridiculous or
unnecessary by others. This phrase is frequently used to avoid
further delay in approval of a document.
For example: "Verifiers MAY wish to track testing mode results to
assist the Signer." [RFC6376]
7. COULD
The phrase "COULD" provides a way for specification authors to
articulate existential possibilities, in order to provide a hint that
might be critical to reliable or secure operation, but without a hard
requirement. The lack of a requirement allows for vendor product
differentiation.
For example: "An implementation could mitigate this race condition,
for example, using timers." [RFC6733]
Barnes, et al. Experimental [Page 4]

RFC 6919 Further RFC Key Words 1 April 20138. POSSIBLE
The phrase "POSSIBLE" describes what some of the working group
members thought of as an edge case that will never happen, but in
practice allows the protocol to work at the most fundamental level.
For example: "It is also possible for the server to send a completion
response for some other command (if multiple commands are in
progress), or untagged data." [RFC3501]
9. MIGHT
The phrase "MIGHT" conveys a requirement in an intentionally stealthy
fashion, to facilitate product differentiation (cf. "COULD" above).
For example: "In the case of audio and different "m" lines for
different codecs, an implementation might decide to act as a mixer
with the different incoming RTP sessions, which is the correct
behavior." [RFC5888]
10. Security Considerations
Traditionally, security requirements in IETF documents have been
expressed with a mixture of requirements words from RFC 2119
[RFC2119] and the phrases used above. The key words in RFC 2119 are
principally useful when threats and mitigations are clear and well
defined. The key words in this document can be applied when the
threat model is ambiguous, and mitigations are unclear or
inconvenient.
11. References11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[RFC0493] Michener, J., Cotton, I., Kelley, K., Liddle, D., and E.
Meyer, "GRAPHICS PROTOCOL", RFC 493, April 1973.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
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